The present invention has been developed for its particular applicability in the processing of semiconductor wafers, such as for making microelectronic devices, where such processing requires precise temperature control and temperature changes. This processing may also require control of the gas mixtures allowed to contact the wafer during the process. Many other types of products and processes involve thermal processing with accurate temperature control of an object, such objects hereinafter referred to as "wafer-like" objects.
In the manufacture of microelectronic devices, such as integrated circuits, flat panel displays, thin film heads, and the like, processing often involves the application of a layer of some material, such as a dielectric, onto the surface of a substrate, such as a semiconductor wafer in the case of integrated circuits. Dielectrics, for example, may need to be baked and then cooled to cure. To prevent oxidation of such a dielectric material, for example, after any processing there of by a baking step, the wafer must be cooled to a certain temperature in an environment of reduced oxygen (an anaerobic environment). Cooling of the wafer also reduces the risk of thermal damage to the wafer transfer mechanism during wafer transfer after processing. The baking and cooling steps must be precisely controlled within exacting temperature constraints to ensure that the selected portions of the dielectric properly set with its desired material properties. Baking and cooling operations for microelectronic devices typically involves cycling a wafer-like object through a desired temperature profile in which the object is maintained at an elevated equilibrium temperature in a controlled environment, cooled to a relatively cool equilibrium temperature, and/or subjected to temperature changes of varying rates (in terms of .degree. C./s) between the equilibrium temperatures. To accomplish baking and cooling, previously known bake/chill operations have included separate bake and chill plates that have required the use of a wafer transport mechanism in order to physically lift and transfer the wafer itself from one place to the other. This approach presents a number of drawbacks. First, wafer temperature is not controlled during transfer between the bake and chill plates. Second, the overall time required to complete the bake/chill process cannot be precisely controlled because of the variable time required to move the wafer to and from the respective plates. Third, the required movement takes time and thus reduces the throughput of the manufacturing process. Fourth, the cost of equipment is higher than necessary because the apparatus requires extra components to handle the wafer during transport from plate to plate. Fifth, the mechanical move from plate to plate introduces the possibility of contaminating the wafer. Sixth, the wafer is exposed to atmospheric oxygen while it is at elevated temperatures, increasing the risk of oxidation. Seventh, the wafer transfer mechanism is exposed to elevated temperatures, reducing its reliability and/or increasing the complexity and expense of its design.
To overcome these deficiencies, a combination bake/chill apparatus has been previously developed by the assignee of the present invention, which is described in copending U.S. patent application Ser. No. 09/035,628, filed Mar. 5, 1998 and entitled "Combination Bake/Chill Apparatus Incorporating Low Thermal Mass, Thermally Conductive Bakeplate", the entire disclosure of which is incorporated herein by reference. That combination bake/chill apparatus includes a low thermal mass, thermally conductive bakeplate to support a wafer during both its baking and chilling operations. With the wafer on one surface of the bakeplate, the other surface of the bakeplate is selectively brought into or out of thermal contact with a thermally massive chill plate so as to switch between baking and chilling operations. In one version, the bakeplate can rest on top of the chill plate during chilling, and one or both of the components is moved to separate them during baking. The bakeplate can heat a wafer by direct conduction of heat generated by the bakeplate to the wafer, while chilling requires heat transfer from the wafer through the bakeplate (which is not heated during the chilling operation) to the chill plate by conduction, which itself is preferably artificially cooled. Both the bake and chill plates are operatively supported within a housing defining a thermal processing chamber. In particular, the housing is formed as a cylinder comprising a cylindrical side wall, a flat top wall, and a flat bottom wall through which various control components extend. The side wall is split so that the top and bottom walls are relatively movable from one another to provide access within the process chamber for loading and unloading wafers.
In developing the present invention, it was discovered that thermal uniformity of a wafer-like object within such a processing chamber is significantly affected by the design and make-up of the process chamber itself. That is, the components making up the processing chamber as well as the components within the chamber, such as for supporting, heating and cooling a wafer-like object, significantly affect the temperature of the wafer-like object throughout its surface area. This is particularly true where such a wafer-like object is to be uniformly heated at relatively high temperatures, e.g., above 200.degree. C. and as high as 450.degree. C. or more. Newer polymers and coatings for semiconductor wafers cure at temperatures of between 350.degree. C. and 450.degree. C., for example. However, as noted above, precise temperature achievement of the entire surface area of a wafer-like object may be required for effective curing or processing. Such thermal uniformity being required in spite of the fact that such a processing chamber should advantageously be designed as a combination baking and cooling apparatus. That is, thermal uniformity is desired even where a wafer-like object is to be heated and cooled within the same chamber. Thus, the structure defining the process chamber and its internal devices not only affect the uniformity of the thermal processing that is conducted on a wafer-like object, they also are subject to cyclical heating and cooling. In general, thermal uniformity in processing a wafer-like object is a function of the relative thermal uniformity of the chamber and its components. So, to achieve good thermal uniformity, such as during a baking step, the process chamber housing and components should be together brought within a desired temperature range. But, as a result of a subsequent cooling operation, the entire chamber and components would be cooled, or at least its temperature uniformity would be compromised. In any case, cycle times would be lengthened in that the achievement of thermal uniformity of a next heating process would require greater time to assure a subsequent achievement of sufficient temperature uniformity of the process chamber.
In developing the present invention, it was also discovered that the gases contained within the processing environment of a baking and cooling apparatus during both steps should be controlled for enhancing the development of the desired material properties.